1. Field of the Invention
The present general inventive concept relates to an electrophotographic image forming apparatus, and more particularly to a fusing unit and an image forming apparatus having the same which fuses a visible image to a printing medium.
2. Description of the Related Art
An image forming apparatus is an apparatus that prints a black and white image or a color image on a printing medium, e.g., paper, according to an inputted image signal, for example, a laser printer, an ink-jet printer, a copying machine, a multi-function printer, a fax machine, etc. An image forming apparatus is classified as an electrophotographic type device in which light is irradiated to a photosensitive body to form an electrostatic latent image and a developer adheres to the electrostatic latent image to transfer the same onto a printing medium, or an ink-jet type device in which a liquid type ink is ejected onto a surface of a printing medium according to an image signal.
Of the two device types, the electrophotographic image forming apparatus is configured such that a surface of a photosensitive body is charged with a predetermined electric potential, a light beam is scanned to the photosensitive body to form an electrostatic latent image due to generation of an electric potential difference, and a developer, i.e., a toner, adheres to the electrostatic latent image to form a visible image. The visible image formed on the photosensitive body is transferred onto the printing medium, and is fused to the surface of the printing medium.
In order to fuse the visible image formed by the developer to the surface of the printing medium, the electrophotographic image forming apparatus has a fusing unit which applies heat and pressure to the printing medium onto which the visible image has been transferred.
The fusing unit generally includes a heating roller which generates heat of a high temperature, and a press roller which is mounted so as to closely contact the heating roller. The heating roller includes a heat source such as a lamp, a heating element or the like, an aluminum pipe provided around the heat source, and a release layer provided on the surface of the aluminum pipe. The press roller is provided with an elastic layer on its outer surface, and is in close contact with the heating roller. The press roller can be separated from the heating roller to remove the printing medium if it becomes jammed between the heating roller and the press roller. During the printing operation, the visible image-transferred printing medium receives heat and pressure while passing between the heating roller and the press roller, and the visible image is fused to the surface of the printing medium.
The fusing unit is set so that the heating roller and the press roller closely contact each other at a pressing force adequate for a thickness of a commonly-used printing medium (e.g., A4 paper). When a printing medium, e.g., an envelope, which is thicker than the common printing medium passes through the fusing unit, the envelope may become wrinkled or crumpled due to the excessive fusing pressure.
To cope with this problem, when printing the printing medium which is thicker than the common printing medium, the fusing pressure between the heating roller and the press roller should be decreased. However, the conventional image forming apparatus is difficult to adjust in use such that a user manually adjusts (decreases or increases) the pressing force between the heating roller and the press roller according to the kind of printing medium (the relatively thicker printing medium or the common printing medium) to be used.
Korean Patent Laid-Open Publication No. 2007-0012191 (published on Jan. 25, 2007) discloses a fusing unit of an image forming apparatus is capable of automatically adjusting the pressing force between the heating roller and the press roller.
But, such a fusing unit is constituted such that a press lever supporting the press roller is elastically supported by a compression spring, and a cam operated by a driving unit directly presses the press lever, thereby automatically adjusting the pressing force between the press roller and the heating roller.
However, such a fusing unit has a problem such that a relatively large force is necessary to move the press lever when adjusting the pressing force between the heating roller and the press roller, because of difficulty of directly pressing the press lever which is elastically supported by the compression spring.
Further, because the press lever supporting the press roller is structured to move automatically, when the printing medium is jammed between the heating roller and the press roller, it is difficult to manually separate the heating roller and the press roller form each other. As a result, it is not easy to remove the jammed printing medium.